1. Field of the Invention
The present invention relates to an ink jet printing apparatus, and more specifically, to an ink jet printing apparatus that executes printing by scanning a printing head in two directions.
2. Description of the Related Art
With the recent spread of personal computers, word processors, facsimile machines, and the like to offices and homes, printing apparatuses based on various printing systems have been provided as information output equipment for the above equipment. In particular, printing apparatuses such as printers which are based on an ink jet system can be relatively easily adapted to execute color printing using plural types of inks. The ink jet printing apparatus has various advantages; for example, it makes only a low noise during operation, can achieve high grade printing on a variety of print media, and is small in size. In this respect, the printer based on this system and the like are suitable for personal use at office or home. Of these ink jet system-based printing apparatuses, a serial type in which a printing head reciprocates to perform printing to a printing medium is very popular because it is inexpensive and can print high grade images.
In spite of its relatively low costs, the serial type printing apparatus is desired to exhibit a higher performance. The printing performance is typified by image quality or image grade, and printing speed.
One of factors that determine image quality or the like is the type of ink. In general, the use of more or appropriate types of inks allows a higher-quality image to be printed. The inks can be classified into dye inks, pigment inks, and the like on the basis of coloring materials used for the inks, or dark and light inks on the basis of the concentration of the coloring materials, or a special color such as orange, red, blue inks, and the like on the basis of ink colors.
Well-known printers use, for example, six types of inks including a dye black ink, a dye yellow ink, a dark and light dye magenta inks, and a dark and light dye cyan inks, or four types of inks including a pigment black ink, a dye yellow ink, a dye magenta ink, and a dye cyan ink. The former apparatus focuses on the output to gloss printing media of photographic images of high quality inputted using a digital camera, a scanner, or the like. The latter apparatus focuses on the high-grade output to ordinary paper of black lines such as black letters and charts.
In general, to obtain a high optical reflection density for black, pigment coloring materials such as carbon black are used to perform printing to an ordinary paper rather than using dye color materials as described above. This is because the pigment is dispersed in the ink and because when this ink is applied to the ordinary paper, the dispersion becomes unstable to cause coagulation, resulting in the effective coverage of the surface of the printing medium. Further, when the ink has a surface tension of about 40 dyne/cm, this prevents the ink from bleeding along fibers in the ordinary paper. Such ink designs enable the printing of letters and lines having a high contrast with respect to the surface of the paper as well as sharp edges. On the other hand, the dye dissolves in the ink at a molecular level, where as the pigment is dispersed in the ink and thus has relatively large coloring material grains. Thus, the pigment cannot pass through a gloss layer in the surface of a glossy printing medium. The pigment accumulates in the surface of the gloss layer to reduce the glossiness.
Thus, when performing printing to a gloss printing medium, the above printing apparatus using a pigment black ink often expresses a black component of an image by using what is called a process black composed of three color inks, a dye yellow ink, a dye magenta ink, and a dye cyan ink, instead of using a pigment black ink. However, to improve the contrast of a black image in a print, it is more preferable to use a dye black ink than to use the three-color inks. In this case, only the dye black ink is used, thus enabling a reduction in the amount of ink applied per unit area of a printing medium. This prevents problems such as ink bleeding. Further, if a gray level is to be expressed in a print image, dots for a color of a relatively high gray level are generally formed by applying a black ink as well as a cyan, magenta, and yellow inks.
In this manner, combinations of various inks are used depending on the type of images to be printed or printing media used. For example, when ordinary paper is important, the apparatus is configured to use a pigment black ink. If gloss printing media are important, the printing apparatus uses a dye black ink.
In contrast, Japanese Patent Application Laid-open No. 11-001647 (1999) describes a configuration focusing on both ordinary paper and gloss printing media. According to this document, the configuration has printing means for a pigment black ink and printing means for a dye black ink. It does not use the pigment black ink but only the dye black ink to perform printing to printing media that have a gloss layer and an ink receiving layer and that are incompatible with the pigment black ink. It uses the pigment black ink to perform printing to the ordinary paper. In this manner, this configuration can print a high-quality or -grade image on both ordinary paper and gloss print media.
Bidirectional printing is known as a configuration that can improve the printing speed, belonging to the printing performance. With this printing system, in a serial type printing apparatus, the printing head is first scanned in a forward direction for printing. Then, paper is fed by a predetermined amount, and printing scan is subsequently executed again by moving the printing head in a backward direction. This printing system achieves an approximately double printing speed or throughput compared to unidirectional printing in which printing is executed during forward scanning, where as it is not executed while the printing head is moving in the backward direction. Other known printing systems include what is called one pass printing in which one scan completes printing of a scan area of a width equal to the arrangement width of ejection openings in the printing head, and what is called multi-pass printing in which printing is completed by a plurality of scans between which paper feeding is interposed. The above bidirectional printing system can also achieve the one pass printing and multi-pass printing. If the one pass printing is executed using the bidirectional printing system, the printing speed or throughput can be maximized.
The bidirectional printing system is effective means in improving the printing speed or the like as described above. However, this system is known to vary colors with scan areas, leading to non-uniform colors or color drifts in a printed image. This is because the application order of the color inks differs between the forward and backward directions of the bidirectional printing. In the printing apparatus, ejection opening rows for the respective color inks are commonly arranged in the scanning direction. However, in this case, the application order may be reversed between the forward scanning and the backward scanning depending on the arrangement of the ejection opening rows.
If dots of a predetermined color are to be formed by applying (ejecting) plural types of inks so that these inks are superposed on a pixel, inks applied to a printing medium earlier more favorably develop their colors. This is because the inks applied to the printing medium earlier easily color the material in a layer closer to the front surface of the printing medium, while the inks applied to the printing medium later less easily color the material in the front surface of the printing medium and permeates deeper through the printing medium in its thickness direction before they are settled. This phenomenon is significant if the ink receiving layer is composed of coat paper consisting of silica. However, it also occurs on ordinary paper or gloss printing media having a gloss layer formed in their front surface and an ink receiving layer formed inside the gloss layer.
Japanese Patent Application Laid-open Nos. 2000-318189 (for example, FIG. 6) and 2001-096771 (for example, FIG. 5) describe a configuration that can avoid non-uniform colors or the like attributed to the application order of inks. In this configuration, two nozzle rows are provided for the respective color inks and arranged symmetrically with respect to an axis orthogonal to the scanning direction.
These documents disclose the configuration in which nozzle rows c1 and c2 for a cyan ink, nozzle rows m1 and m2 for a magenta ink, and nozzle rows y1 and y2 for a yellow ink are each arranged symmetrically with respect to a predetermined axis of symmetry orthogonal to the scanning direction of the printing head, for example, as shown in FIG. 16. In this configuration, to form an ink dot for each pixel, the inks are ejected (applied) in order of c1, m1, y1, y2, m2, and c2 in the forward scanning direction. The inks are ejected (applied) in order of c2, m2, y2, y1, m1, and c1 in the backward scanning direction. This enables the inks to be applied or superposed on one another in the same order between the forward scanning and the backward scanning (c←m←y or y←m←c). In other words, the inks are applied in two different orders between the forward scanning and the backward scanning. As a result, for dots formed by superposing the cyan, magenta, and yellow inks on one another, the application or superimposition order remains unchanged regardless of the scanning direction. Alternatively, two types of dots can be formed for each pixel on the basis of the different application orders. These dot formations can reduce the non-uniformity of the colors attributed to the bidirectional printing.
On the other hand, as shown in the same figure, the relationship between nozzle rows k1 and k2 for a black ink and the other ink nozzle rows is such that the inks are ejected in order of k1, k2, c1, m1, y2, m2, and c2. In this case, the super position order of the black ink and the other inks varies depending on the scanning direction. If image data to be printed forms dots using only the black ink, the super position of this ink on the other inks described above does not occur. However, for example, in expressing a gray tone, the black ink may be superposed on another color ink such as cyan to form dots in order to smooth a variation in gray level. In this case, the application or superimposition order of the black ink and the other color inks may vary depending on the scanning direction. This may result in non-uniform colors.
This will be described in further detail in connection with under color removal commonly executed as image processing for generation of the above data.
FIG. 17 illustrates an example of an under-color removal process. This figure indicates the relationship between the gray level and the respective output levels of process black obtained using a cyan ink, a magenta ink, and a yellow ink and of black obtained using a black ink. In the illustrated under-color removal process, when the gray level is relatively low (0 to 187), only the cyan ink, magenta ink, and yellow ink are outputted so as to form an image using the process black. Then, the black ink starts to be used at a predetermined medium density (187) in the gray level. At the maximum density level, the data is outputted so as to use only the black ink.
The process black ink is used when the gray level is relatively low because the cyan ink, the magenta ink, and the yellow ink are lighter and give a less significant granular impression than the black ink, thus enabling a smooth gray level expression. Both process black ink and black ink are used when the density is higher than the medium density (187 or more) because the formation of a black image using the black ink requires less inks to be applied to a printing medium than the printing of a black image using the process black ink, thus preventing problems such as the overflow of the inks during printing. Furthermore, the use of the black ink enables the printing of a black image with a higher optical reflection density and a higher contrast.
Thus, when the gray level is between the medium density and the maximum density, the black ink and the process black ink are superposed on each other. The conventional printing head configuration shown in FIG. 16 can of course form such dots. In this case, the process black ink and the black ink are unlikely to be superposed on each other close to the medium and maximum densities. Consequently, the varying ink application order attributed to the bidirectional printing is unlikely to cause non-uniform colors.
However, between the medium density level, at which the black starts to be used, and the vicinity of the maximum density level, at which only the black ink is used, there exists an area in which dots are formed with the cyan ink, magenta ink, and yellow ink, constituting the process black, and the black ink being superposed. In an image of a density level within this area, the non-uniformity of the colors may be significant which is attributed to the application order varying depending on the scanning direction.
The inventors of the present invention have found out that a dot formed by superposing one, two, or all of the cyan ink, magenta ink, and yellow ink and the black ink is differently colored depending on an overlapping manner, that is, the order of superposing the black ink in relation to the other color inks, or to which color ink the black ink is superposed to be adjacent. Specifically, in the conventional arrangement of the ejection openings for the black ink and other color inks such as the one shown in FIG. 16, the overlapping manner may vary markedly between the forward and backward directions of the bidirectional printing. Consequently, a dot formed by superposing the black ink and the other color inks may be differently colored between the forward direction and the backward direction. This results in non-uniform colors.
A configuration has been proposed in which like the nozzle rows for the cyan, magenta, and yellow inks, the nozzle rows for the black ink are symmetrically arranged in order of, for example, k1, c1, m1, y1, y2, m2, c2, and k2. However, in this case, supply liquid chambers must be provided to supply the nozzle rows k1 and k2 with the corresponding inks. This increases the size of the printing head. In contrast, with two adjacent nozzle rows, only one ink supply liquid chamber is required, suppressing an increase in size.